Earthing

[Jel]

Hi guys,

Doing some work in a rural area. The client is having an office built behind his house. It consists of a bathroom/shower room, kitchen, and office space.
The wants electrical feed taken from the house and it having a trench dug to run all services.
The house is on a TT, and has it owns earth rod.
Was going to run a 2 core 25mm SWA from a 80a wylex isolator in main house and run that straight into CU in office.
Questions:
1: Can i use the earth from the house, through the sheathing of SWA, if the Ze is less than 200Ω when tested at the office end, or do i need to install my own earth rod?
2: Does the SWA need to be a certain distance in the trench away from other services?
3: Is 0.6m still the correct depth for a buried cable?

Thanks

Jel

 

[nickblake]

and its 3 yes' from me ..

 

[mark80]

I think personally the main consideration you should be giving to this design is the maximum demand. It appears from your post that the house may have just a single phase supply? If so, you need to assess the maximum demand for both the house and new office and ensure the main fuse is rated appropriately.
On the other points
1 - I'd earth the SWA at both ends and have the new office with its own earth rod aswell
2 - Yes the cable needs to be a minimum distance apart from other services but I havent the regs to hand so you will have to find out
3 - There is no specified depth, however you should assess the area in which it will be buried and what its use is etc and decide from there.

 

[Lighthouse Phil]

Why not run a 3 core SWA. ??

 

[Knobhead]

Earth both ends of the cable and if possible run a 3 core. With a TT system the wider the earths are spread the better. The more earth points the better so long as they’re adequately linked.

Just in case I’ve got my tin helmet out

View attachment 10423

 

[Deleted member 9648]

You can either use the existing rod....(<200 ohms),or add another and link....There will no doubt be a hue and cry on here about Ra values being as low as possible,all well and good,but unless you achieve TN values you will still be reliant on RCD's for earth fault protection.
Personally I would use the house rod if it's Ra value is acceptable,and protect the sub..(3c) with a 100ma time delayed RCD...and use 30ma RCD protection at the office end.
(tin hat already shell damaged beyond repair)

 

[Engineer54]

Earth both ends of the cable and if possible run a 3 core. With a TT system the wider the earths are spread the better. The more earth points the better so long as they’re adequately linked.

Just in case I’ve got my tin helmet out

View attachment 10423

Pretty much right Tony, though there's little gain in the distance between rods/electrodes, so long as the fields between rods don't overlap your fine.

I see the great defender of our great UK TT systems is off on a run again...lol!! Now it's, don't even try to improve what's already there, ...Ra stability ...what's that then!!

 

[GT1]

The regs are very helpful when it comes to depth of cables in trenches.
"as deep as necessary".......

 

[Engineer54]

The regs are very helpful when it comes to depth of cables in trenches.
"as deep as necessary".......

Well, you won't go far wrong if you use the traditional and generally accepted 60cm burial depth for direct buried cables.

 

[Jel]

Thanks everyone. So just clarify running a 2 core swa will be ok and to use the sheaf of swa as a earth?

 

[Engineer54]

Thanks everyone. So just clarify running a 2 core swa will be ok and to use the sheaf of swa as a earth?

So, what are you going to do then, just use the existing Rod as your earthing arrangement for for the new building??

 

[Lighthouse Phil]

For the extra few quid, I'd still use a 3 core SWA, and a new rod.

 

[Deleted member 26818]

2core or 3core SWA, it doesn't make much difference.
However, if the new building is any real distance away from the house, there could well be a difference in potential between the local earth at the new building and the earth at the house.
As such I would install another rod at the new building.

 

[Engineer54]

2core or 3core SWA, it doesn't make much difference.
However, if the new building is any real distance away from the house, there could well be a difference in potential between the local earth at the new building and the earth at the house.
As such I would install another rod at the new building.

Could well be if the soil type is crap. The thing is, no-one knows what the condition of this existing rod is, could well be a cut off rod, because the installer hit a rock or whatever.
Maybe giving a reading under the magical 200 ohms too, during wet weather, but maybe not when that wet soil freezes, or dries out.

For the cost of another rod at the new building, and ensuring the 2 rod positions are linked via the SWA or a 3rd core, it really doesn't make much sense not too to install one!! That and a S type RCD at the house is a far better option, than relying on an unknown...